In railway and rapid transit operations, it is a common practice to provide various train control signals at interlockings which employ insulated joints and impedance bonds to establish the length of track circuits and to precisely define the boundaries of the track circuit. Each of the impedance bonds must be capable of accommodating audio frequencies for both train detection and cab signals, as well as for handling propulsion current in electrified territory. Generally, insulated joints are unnecessary except in those areas around interlockings. The impedance bonds normally include a heavy wire multiple turn winding connected across the track rails and a center tap connector for permitting propulsion current to flow around the insulated joints. In addition, the impedance bonds include a plurality of other windings which are tuned to the frequencies of the train detection and cab signals. In the areas between interlockings, the track circuits are jointless and are commonly as long as 1200 feet. In this jointless territory, the track circuits are typically preshunted 40 or so feet in advance of the impedance bond. Therefore, the cab signal current flows past the bond and through the axles of the oncoming train to maintain a continuous cab signal indication. Because the bond is tuned, the impedance of the bond is relatively high in comparison to the impedance of the 40 feet preshunt of the track before the bond. However, in interlocking areas, the track circuits are relatively short, and it is common to have track circuits which are only 100 feet long. However, it is advantageous, from both an economic and operational standpoint, to attempt to utilize the same types of impedance bonds at interlockings as those used in jointless track areas. Previously, it was reported and verified that if an insulated joint failed there was sufficient leakage cab signal current to falsely signal an approaching train to proceed into the interlocking. For example, when two opposing trains approach an interlocking which is conditioned to make a turn-out move, it is necessary to ensure that a failed insulated joint will not permit the approaching train to enter the interlocking at the opposite end in order to prevent the possibility of a collision.